Depends on the reviewer and whether or not your site gets tagged with certain keywords.
Even if you do get algorithmically banned you may still get manually unbanned. But it really depends on the reviewer and will take time, time of which you will earn no money from adsense.
Be careful to make sure entire forum is "anti-smoke". Just one single thread of a smoker vouching for certain tabacco product (that's less addicting) and you are done. Which I believe may be the case for those gradually quitting products.
Google will warn you to fix it, but it will be finding a needle in a sea of haysack. Unless you can go over 1 million threads to single out those ones within like 3 business days?, you're done.
Make your own judgement call.
I would personally slap it on, keep options open when unrealistic requirements come if they do come, then jump ship to another network.
Forum is always tricky and you will have to work hard to hand mod every post to ensure program compliance.